An image forming apparatus (such as a copier) which forms an image on a print medium (such as a sheet) includes a fixing device which fixes toner, transferred to the print medium, by thermocompression bonding. The fixing device includes a fixing roller and a pressure roller which are opposed to each other, and the fixing roller incorporates one heater or a plurality of heaters.
The surface of the fixing roller is heated by the heater to a predetermined target temperature, and the heated fixing roller and the pressure roller sandwich and press the print medium. Thus, the toner is fixed on the print medium. Here, the fixing roller contacts a surface on which the toner is transferred, and the pressure roller contacts another surface where the toner is not transferred.
The temperature of the fixing roller is an important factor for determining whether or not the toner is fixed on the print medium finely. The heater for heating the fixing roller is usually a heater which converts electric energy to heat energy. Therefore, the temperature control of the fixing roller is carried out by controlling electric power supplied to the heater. Feed back control is usually used to control the temperature of the fixing roller. The feed back control is to detect the temperature of the fixing roller, compare the detected temperature with the target temperature and increase or decrease the electric power supplied to the heater.
There are various methods for detecting the surface temperature of the fixing roller. A conventionally used method is to cause a temperature detecting element (such as a thermistor) to contact a region of the fixing roller which region contacts the print medium. According to this method, a resistance value of the temperature detecting element which value corresponds to the temperature is detected by a voltage, and the surface temperature of the fixing roller is obtained from the detected voltage.
In recent years, the image forming apparatus has been realizing high speed and colorization, and the rotation speed of the fixing roller has been increasing. As a result, the conventional method for causing the temperature detecting element to contact the fixing roller causes scratches or damages on the surface of the fixing roller even if its contact pressure is reduced. This poses problems where the toner is not fixed uniformly and the quality of an image formed on the print medium deteriorates.
Here, Tokukaihei 11-133796 (Japanese Unexamined Patent Publication 11-133796 (published on May 21, 1999)) discloses a method for providing the temperature detecting element (such as the thermistor and a thermocouple) in a noncontact manner with respect to a portion of the fixing roller which portion contacts a recording sheet. According to this method, it is possible to detect the changes in the temperature of the fixing roller quickly, and also possible to prevent scratches and damages caused by the contacting of the temperature detecting element. However, in order to directly detect the temperature of the fixing roller, the temperature detecting element needs to be placed near the fixing roller. This poses a problem of the heat resistance of the temperature detecting element.
Moreover, widely used as a noncontact-type temperature detecting element is a thermopile element. The changes in an output signal of the thermopile element are small as compared with the changes in the temperature detected. Therefore, if the compensation is not carried out with respect to the reference temperature near the thermopile element and the amplification is not carried out, the thermopile element generates the output signal containing a large amount of noise. Therefore, it is impossible to detect the temperature accurately. On this account, it is necessary to provide the thermopile element and an amplifier near the fixing roller that is a heat source. However, these members are weak against heat. This especially poses a problem of the heat resistance.
Here, Tokukaihei 11-223555 (Japanese Unexamined Patent Publication 11-223555 (published on Aug. 17, 1999) discloses a noncontact-type temperature sensor which detects the temperature of the fixing roller by detecting the infrared radiation, emitted from the fixing roller, by a thermistor element. In this sensor, the thermistor element is provided on a film which absorbs the infrared radiation, the infrared radiation film absorbs the infrared radiation emitted from the fixing roller, and the heat generated by the absorption is detected by the thermistor element. Thus, by using the thermistor element as the temperature detecting element, it is possible to alleviate the problem of the heat resistance. As a result, the conventional use condition of up to 100° C. is improved to 150° C., and it becomes possible to provide the noncontact-type temperature detecting element in the fixing device.
Note that in this method, the temperature detecting element can detect not the temperature of the surface of the fixing roller itself but a relative temperature difference between the temperature detecting element and the fixing roller. Here, generally, a compensation temperature detecting element which detects a temperature around the temperature detecting element is further provided at the periphery of the temperature detecting element which detects the infrared radiation, and an output value of the temperature detecting element which detects the infrared radiation is compensated by an output value of the compensation temperature detecting element.
Moreover, Tokukai 2003-302288 (Japanese Unexamined Patent Publication 2003-302288 (published on Oct. 24, 2003)) also discloses a temperature detecting means (i) which includes a temperature detecting element for detecting the infrared radiation and a compensation temperature detecting element and (ii) which is configured so that the difference between the output voltage of the temperature detecting element and the output voltage of the compensation temperature detecting element becomes constant when the temperature of the fixing roller is in a desired temperature range. With this, it is possible to detect excessive temperature rising of the fixing roller.
However, the conventional noncontact-type temperature detecting means disclosed in Tokukaihei 11-223555 (Japanese Unexamined Patent Publication 11-223555 (published on Aug. 17, 1999) and Tokukai 2003-302288 (Japanese Unexamined Patent Publication 2003-302288 (published on Oct. 24, 2003)) cannot carry out the temperature control of the fixing roller accurately. According to the techniques in Tokukaihei 11-223555 (Japanese Unexamined Patent Publication 11-223555 (published on Aug. 17, 1999) and Tokukai 2003-302288 (Japanese Unexamined Patent Publication 2003-302288 (published on Oct. 24, 2003)), the output value of the noncontact-type temperature detecting means is analogically compensated by the output value of the compensation temperature detecting means, and the temperature of the fixing roller is detected. However, when various disturbances occur, the detected temperature of the fixing roller may become inaccurate. For example, in the case of the temperature detecting means disclosed in Tokukai 2003-302288 (Japanese Unexamined Patent Publication 2003-302288 (published on Oct. 24, 2003)) discloses that the error of about 17° C. occurs depending on the ambient temperature detected by a compensation thermistor. Thus, according to the conventional techniques, if the environmental conditions at the periphery of the noncontact-type temperature detecting section change, there are problems in that the error of the detected temperature becomes large and this affects the temperature control.
Moreover, Tokukai 2003-149981 (Japanese Unexamined Patent Publication 2003-149981 (published on May 21, 2003)) proposes that the temperature of the fixing roller is obtained by calculation using a detection value of a temperature sensor. However, as a matter of reality, it is difficult to accurately express the relation between the detected value and the temperature by a mathematical formula. Especially, in light of shortening of calculation time, an approximation formula is used in many cases. In such a case, a calculation result is often largely different from the actual temperature.